I understand that there is no definite answer to this question.
In my opinion the Etudes, particularly Winter Wind, Op.25 No.6 and Op.10 No.1 are the hardest of his compositions. Others include his Ballades (No.4), Preludes (op. 28 no. 24) and his extremely long Sonatas. I have played all of this except his Preludes, so I do not know how actually hard they are, only from hearing them.